Your transfer is failing because Twilio or Telnyx is blocking the caller ID when you set it to “user’s number. When Retell tries to transfer the call, the carrier checks. Does this account own or verify this caller ID? If no then the call is rejected. That’s why it works with a retell-purchased Twilio number, but fails when you bring your own SIP trunk (Twilio or Telnyx). I can help with a quick fix if you don't mind